There is known a method for forming a silver element, such as a metal thin film, on a substrate by applying or printing a liquid or paste ink containing metal particles on a substrate, followed by heating. The metal used here may be gold, silver, copper, or aluminum, with silver being generally used as a wiring material. Ink with silver usually contains silver metal dispersed in a solvent. The ink is patterned on an interconnection substrate, and silver metal in the ink is sintered to form wiring. It is known that, when used as an electrically conductive material, silver metal may be sintered at a lower temperature, making use of melting point depression caused by the fine size of the dispersed silver metal particles. On the other hand, silver metal particles of such a fine size as to cause melting point depression are prone to contact and aggregate with each other. For preventing aggregation, a dispersant is required in the ink (see, for example, Patent Publication 1).
Formation of a silver element with the ink containing a dispersant on a substrate by heating may result in residual impurities originated from the dispersant. In view of this, it is usually desired to remove the impurities by heating at a high temperature, such as 150° C. or higher.
A general method of forming a silver element on a substrate includes, for example, heating a silver salt composed of an inorganic acid and silver, such as silver nitrate, in the presence of a reducing agent and a dispersant. With this method, however, a residual acid component originated from the silver salt may result, and high-temperature heating is required for removal of the dispersant.
A method of forming a silver element with a silver salt containing an organic acid, instead of the inorganic acid, has also been reported. The organic silver proposed are, for example, silver salt of long-chain carboxylic acid (Patent Publication 2) or silver α-ketocarboxylate (Patent Publication 3).
However, quick decomposition of the organic silver calls for heating treatment at not lower than 150° C., and a low silver content of the organic silver causes difficulties in formation of a silver element with excellent flatness and adhesivity.
There have recently been made active attempts to form a silver element on a transparent resin substrate. A transparent resin substrate generally has a lower softening point compared to glass or the like, so that formation of a silver element on a transparent resin substrate is desirably made by a low-temperature heat treatment at lower than 150° C. The low-temperature heat treatment requires a lower thermal decomposition temperature of the silver salt. In view of this, silver β-ketocarboxylate of a particular structure is proposed which has a decomposition temperature of lower than 150° C. (Patent Publication 4).
However, this silver β-ketocarboxylate of a particular structure, which is a silver salt composed of a monofunctional carboxylic acid and silver, has a low silver content, and leaves a larger amount of residual organic component when heated. Thus, the resulting silver element provides low flatness and adhesivity to a substrate. On the other hand, decomposition and evaporation of the organic component requires long heating time, which lowers the production efficiency.
A silver salt composed of silver and malonic or oxalic acid has a higher silver content, as malonic and oxalic acids are dicarboxylic acids. However, decomposition of these silver salts in a short time requires high-temperature heating at not lower than about 210° C., so that the low-temperature heat treatment is hard to be realized.
There has been reported that silver acetonedicarboxylate, which is a dicarboxylate, may be used as an intermediate in the synthesis of an acetonedicarboxylate (Non-patent Publication 1).
This publication makes, however, no disclosure about the thermal decomposition characteristics of silver acetonedicarboxylate. In this regard, the thermal decomposability of silver acetonedicarboxylate, which has a higher silver content, was evaluated, to find that the thermal decomposition temperature was not lower than 150° C., which leads to an assumption that silver acetonedicarboxylate is hard to be used in a low-temperature sintering ink.
Patent Publication 1: JP-2005-60824-A
Patent Publication 2: JP-2005-298921-A
Patent Publication 3: JP-2004-315374-A
Patent Publication 4: JP-2008-159535-A
Non-patent Publication 1: Jornal fur praktische Chemie. Band 312 (1970), pp 240-244